Sure you can. Voltage is the only way to discern SOC.
You just can't measure accurately under load. You have to measure open circuit
voltage.
www.mdpi.com/1996-1073/9/11/900/pdf

This paper mainly discusses the higher-voltage Li-Ion cells, which have a larger
voltage change between full and empty.

For the LiFePO3 cells being discussed here, the voltage change is much smaller.
As Bill says, the change is so small that it can't be reliably measured outside
of laboratory conditions and precisions instruments. Other factors, such as age,
temperature, manufacturing variations, and the accuracy of the equipment play a
larger factor.